An update. I have pulled the plugs at various times. Everything seems fine.
I do notice a sound (pitch?) change and a slight loss of power sometimes during a long or hard drive. Have pulled over and pulled plugs to check. Seems OK.
I am beginning to wonder whether I should change the plug wires. I've basically had to change everything else provided with this car. Why stop now?
I realize that I could have a coil issue but I'll address that separately.
Thanks to everyone who has provided advice on this.
Marshall